There are many themes in this book. The eNotes summary and study guide for the book has an excellent discussion of the themes. Among the themes that it lists are:

Hospitality. You could also call this generosity. For example, we see that Rat lets Mole come and live with him even though Rat has only met Mole that same day. He allows Mole to stay with him a long time.

Forgiveness. Characters are forgiven for bad things they do, whether big or small. Even the weasels are forgiven when they show that they feel sorry for what they have done.

Compassion. Characters tend to understand the others' needs and act to try to help others. For example, Badger, Mole and Rat try to help Toad get over his mania for driving.